The present invention relates to a structure for mounting a component to a printed circuit board and to a method for doing the same, and more particularly relates to such a mounting structure and such a mounting method which are particularly adapted for aiding with the mounting of a so called surface mounted electronic component to a printed circuit board.
In recent times, the mounting of electronic parts to printed circuit boards according to the so called surface mounting technique has come more and more into favor; this is because of the lowering of cost provided with such a technique, and because of the convenience provided thereby. Further, as a refinement of such a surface mounting system, the so called double surface mounting technique in which electronic components are mounted to both the sides of a single printed circuit board has also started to come into vogue; this double surface mounting technique provides further convenience and lowering of cost, and allows beneficial increase of the density of electronic components upon the printed circuit.
In more detail, as shown in illustrative side view in FIG. 6 of the accompanying drawings, an electric component generally designated as 41 typically has terminals 42 which, for surface mounting, are typically bent through right angles so as to be constituted as leg portions 42a and foot portions 42b. And the foot portions 42b are typically soldered to portions 44 of a printed circuit pattern formed on a printed circuit board 43. This soldering of the terminal foot portions 42b to the printed circuit portions 44 is typically performed by coating solder paste over said printed circuit portions 44, then placing the foot portions 42b of the terminals 42 against said solder paste coated printed circuit portions 44 so that the electronic component 41 is at least somewhat held in place thereby at a typical distance such as shown by A1 in the figure from said printed circuit board 43, and then passing the whole assembly through a heating oven so as to melt the solder paste and so as to bond said terminal foot portions 42b to said printed circuit portions 44. In fact, typically, a considerable number of electronic components such as 41 are fixed to the printed circuit board 43 at the same time in this manner, with one pass through the heating oven being performed.
However, this type of mounting structure, and this mounting method, are prone to problems, as follows. First, since the vertical distance A1 from the bottom of the electronic component 41 (the portion thereof closest to the printed circuit board 43) to said printed circuit board 43, is typically quite greatdue to the typically considerable length of the terminal leg portions 42a, thereby the stability of the electronic component 41 is not very good, and the center of gravity thereof is rather far from the printed circuit board 43. Accordingly, during the soldering process, at which time any adhesive effect provided by the solder paste holding the terminal foot portions 42 to the printed circuit portions 44 is inevitably substantially released, there is a great risk that the electronic component may undesirably become displaced from its position with the terminal foot portions 42 in contact with the printed circuit portions 44, and this can cause poor connection between these elements, since in such a case said terminal foot portions 42 will come away from contact with said printed circuit portions 44 and not be properly soldered thereto.
As an expedient to cope with the above identified problems, it might be considered to provisionally bond the lower surface of the casing of the electronic component 41 to the printed circuit board 43, but this is not practicable with the construction as shown in FIG. 6, since the distance A1 is typically too great for such bonding.
Alternatively, as another expedient to cope with the above identified problems, it might be considered to shorten the length of the terminal leg portions 42a, as shown exemplarily in illustrative side view in FIG. 7 of the accompanying drawings, so as to reduce the vertical distance from the bottom of the electronic component 41 to the printed circuit board 43 to a distance such as shown by A2 in that figure. However, this expedient is fraught with other problems. Namely, first, if the distance from the component 41 to the printed circuit board 43 is so reduced, the problem arises that poor heat dissipation may occur. Furthermore, since in this case the terminal leg portions 42a are so short, it may happen that thermal stress and deformation which may be set up, when the apparatus is being used and the electronic component 41 is generating heat, due to difference between the coefficients of thermal expansion of the casing of said electronic component 41 and of the printed circuit board 43, cannot be properly absorbed. Such thermal stress and deformation, in the worst case, may lead to such force being exerted on the portions 44 of the printed circuit pattern on said printed circuit board 43, that said printed circuit portions 44 become peeled away from the base circuit board 43, and may further become fractured and interrupted. This could lead to failure of the assembly as a whole. Such thermal stress and deformation could also lead to cracking or fracture of the electronic component 41 itself.
Further, the above outlined problem of dissociation that may arise in the case of mounting such an electronic component to such a printed circuit base board is exacerbated in the case of the above described double surface mounting technique in which electronic components are mounted to both the sides of a single printed circuit board, because if the foot portions of the terminals of such components are provisionally attached to both sides of the printed circuit base board, only by solder paste as outlined above, it is almost inevitable that some of the components on the one side or the other of the printed circuit base board will be improperly attached, due to the fact that at least one side of the printed circuit base board cannot be face up.